Set the PTO (or blade switch) to the "off" position.
Set the parking brake to the "on" position.
Sit on the seat and turn on the machine. If it does not turn on, there is something wrong with one of the safety switches.
Open the electrical panel underneath the steering wheel and locate the safety switches.
Pull out the electrical fuses using a fuse puller. Insert a new fuse for each safety switch, one at a time, to test each one. If the mower works after inserting a new fuse, you've located the problem.
Bring your Troy-Bilt to a certified service technician if the problem persists. Safety switches should be repaired by a professional.